1. Field of the Invention
The present disclosure generally relates to well cementing, and more particularly to cement compositions for preventing gas or water migration and methods of cementing a wellbore using such cement compositions.
2. Background of the Invention
Zonal isolation refers to the isolation of a subterranean formation or zone, which serves as a source of a natural resource such as oil, or water, from other subterranean formations. To achieve isolation of a subterranean formation, a wellbore is typically drilled down to the subterranean formation while circulating a drilling fluid through the wellbore. After the drilling is terminated, a string of pipe, e.g., casing, is run in the wellbore. Next, primary cementing is typically performed whereby a cement slurry is placed in the annulus and permitted to set into a hard mass, thereby attaching the string of pipe to the walls of the wellbore and sealing the annulus. Subsequent secondary cementing operations such as squeeze cementing may also be performed.
One problem commonly encountered during the placement of a cement slurry in a wellbore is unwanted gas or water migration from the subterranean zone into and through the cement slurry. Gas migration is caused by the behavior of the cement slurry during a transition phase in which the cement slurry changes from a true hydraulic fluid to a highly viscous mass showing some solid characteristics. When first placed in the annulus, the cement slurry acts as a true liquid and thus transmits hydrostatic pressure. However, during the transition phase, certain events occur that cause the cement slurry to lose its ability to transmit hydrostatic pressure. One of those events is the loss of fluid from the slurry to the subterranean zone. Another event is the development of static gel strength, i.e., stiffness, in the slurry. As a result, the pressure exerted on the formation by the cement slurry falls below the pressure of the gas in the formation such that the gas begins to migrate into and through the cement slurry. When gas migration begins, the cement slurry typically has a gel strength of about 100 lbf/100 ft2. The gas migration causes flow channels to form in the cement slurry. Eventually the gel strength of the cement slurry increases to a value sufficient to resist the pressure exerted by the gas in the formation against the slurry. At this point, the cement slurry typically has a gel strength of about 500 lbf/100 ft2. Likewise, prior to setting of the cement slurry, water may flow or migrate into the slurry and form flow channels. The cement slurry then sets into a solid mass.
Unfortunately, the flow channels formed in the cement during such gas or water migration remain in the cement once it has set. Those flow channels can permit further migration of gas or water through the cement even long after the cement is set. Thus, the cement residing in the annulus may be ineffective at maintaining the isolation of the adjacent subterranean formation. To overcome this problem, attempts have been made to design a cement slurry having a shorter transition time, i.e., the period of time during which gas migration into the slurry can occur, which is typically the time ranging from when the gel strength of the slurry is about 100 lbf/100 ft2 (pound force per hundred square foot) to when it is about 500 lbf/100 ft2. While cement slurries having shorter transition times have been developed, those slurries are typically very expensive to prepare. Further, their transition times are still longer than desired.
A particular challenge is cementing at lower temperatures in a formation having the potential for water or gas migration at shallow hole depths, and in particular using lightweight cement slurries such as foamed slurries, water extended slurries, bead-containing slurries, etc. Cement blends, such as Portland or ultrafine cement blends, have been used with accelerators in an effort to obtain rapid setting of the cement and the development of compressive strength. In situations where the temperature is low, i.e. less than about 40° F., an excessive amount of such accelerators have been employed in an effort to facilitate the rapid setting and strength development of a cement composition. However, the addition of an excessive amount of accelerator has often resulted in cement compositions that rapidly form viscous gels with a premature loss of pumpability. Thus there exists an ongoing need for improved methods of eliminating gas or water migration during well cementing to reduce the risk of compromising zonal isolation. Furthermore, a need exists for cement compositions that balance transition time with the need for rapid development of compressive strength.